Why don't I get a collection item for assembling a collection?

When you assemble a completed collection you get a reward stated below it. It varies and may include coins, experience points and some useful items. As for the item shown next to each collection, it is meant for a visual representing of the fact whether the collection has been assembled or not. When it is activated, the item becomes brighter in colour and a figure below it shows the number of times the collection was completed.